1. Field of the Invention
The present invention concerns a rotor for a gantry of a computed tomography apparatus, as well as a manufacturing method for such a rotor.
2. Description of the Prior Art
Computed tomography apparatuses enable the reconstruction of three-dimensional slice or volume images of an examination region for diagnostic purposes. The reconstruction of an image ensues on the basis of projections of an examination region that are acquired by irradiating a subject with an x-ray fan beam from different projection directions by rotation of an acquisition device so that measurement data are acquired for parallel projections in an angle range of at least 180 degrees plus the fan angle for reconstruction of an image. To achieve the rotation of the data acquisition device, the computed tomography apparatus has a gantry that has a stationary frame and a rotor arranged such that it can rotate by means of a rotating bearing device. The image data acquisition device is mounted on the rotor. The rotor has been produced conventionally as a cast part made of an aluminum alloy AlZn10SiMg and has a rotor wall in the form of an annular disc and a retention ring running along its outer periphery for mounting the components of the acquisition device. The wall thicknesses of such rotors vary between 15 and 20 mm.
To avoid movement artifacts in the reconstructed image that can arise due to patient or organ movements, it is sought to select the time window for acquisition of the projections required for reconstruction to be as small as possible by the use of high rotation speeds. Rotation speeds of 210 R/min are achieved in current computed tomography apparatuses. In the future the rotation speeds are expected to be increased to at least 300 R/min.
Due to a combination of high rotation speed, large rotation radius and high rotation mass, the rotor represents a highly mechanically stressed component that, in addition to accommodating the stresses that are incurred, must also reliably maintain the positions of x-ray tubes and detectors, since position shifts of the components of more than 0.15 mm can lead to a significant degradation of the image quality.
Significant primary requirements for the rotor of a gantry are accordingly not only a high strength to transfer the forces, but also a high rigidity in order to keep deformations of the rotor (and thus the position shifts of the components of the acquisition device) below the allowable limits, given a simultaneously low weight.
An additional thickening of the existing design would be necessary in order to achieve rotation speeds of 300 R/min and more while keeping the same material. The consequence would be a weight increase of the rotor. Components to drive the rotor and the stationary part of the gantry would thereby also have to be adapted to the greater weight. This approach also has the disadvantage of causing a weight and volume increase of the entire gantry.